1. Field of the Invention
This invention relates to a catalyst deterioration diagnosis apparatus and a catalyst deterioration diagnosis method which diagnose the deteriorated state of a catalyst converter disposed for the exhaust gas purification of an internal combustion engine, and which gives warning to a driver.
2. Description of the Related Art
An apparatus for purifying the exhaust gas of an internal combustion engine as is extensively put into practical use is one wherein a ternary catalyst converter (hereinbelow, termed “catalyst converter” or simply “catalyst”) which simultaneously oxidizes CO and HC and deoxidizes NOx is disposed in the exhaust system of the engine, while O2 sensors are respectively disposed on the upstream side and downstream side of the catalyst, and an air/fuel ratio feedback control is performed in accordance with the detection signals of the O2 sensors, whereby an air/fuel ratio can be controlled within a very narrow range in the vicinity of a theoretical air/fuel ratio, and the purifiability of the catalyst disposed in the exhaust system can be held high.
In a case where the catalyst has had its purification efficiency lowered gradually or has been destroyed by the heat of the exhaust gas, sulfur poisoning, or the like, a vehicle in which the engine is installed travels while emitting harmful components, and it is accordingly desirable to take a measure such as the exchange of the catalyst. The driver of the vehicle, however, is difficult of sensing the deterioration or destruction of the catalyst.
In this regard, a catalyst deterioration detection apparatus for an internal combustion engine as proposed in JP-A-5-98949 (Patent Document 1) and a catalyst deterioration diagnosis apparatus proposed in JP-A-7-305623 (Patent Document 2), for example, have been known as apparatuses for deciding (diagnosing) such a deteriorated state of the catalyst.
Each of the apparatuses proposed in the patent documents executes the air/fuel ratio feedback control in accordance with the output signals of the O2 sensors which are respectively disposed on the upstream side and downstream side of the catalyst interposed in the exhaust passage of the internal combustion engine, and it diagnoses the deteriorated state of the catalyst by comparing the output signals of both the O2 sensors.
FIGS. 13A-13E are graphs showing the “output waveforms of the upstream-side O2 sensor and downstream-side O2 sensor during the air/fuel ratio feedback control” in the internal combustion engine in the related art.
During the execution of the air/fuel ratio feedback control, a fuel feed quantity is controlled (in other words, feedback-corrected) by, for example, a proportional and integral control shown in FIG. 13A, chiefly on the basis of the output signal of the upstream-side O2 sensor.
Accordingly, the output signal of the upstream-side O2 sensor cyclically repeats the inversion of rich and lean states with respect to “rich”/“lean” decision voltage as shown in FIG. 13B.
In contrast, on the downstream side of the catalyst, the fluctuation of a remaining oxygen concentration becomes very gentle owing to the O2 storage capability of the catalyst. As shown in FIG. 13C, therefore, the output signal of the downstream-side O2 sensor has a smaller fluctuating width and a longer fluctuating cycle as compared with that of the upstream-side O2 sensor.
However, when the catalyst has been deteriorated, oxygen concentrations do not become considerably different between on the upstream side and downstream side of the catalyst, on account of the lowering of the O2 storage capability.
As a result, as shown in FIGS. 13D and 13E, the output signal of the downstream-side O2 sensor comes to repeat inversion with a cycle approximate to that of the output signal of the upstream-side O2 sensor, in accordance with the degree of the deterioration of the catalyst, and it comes to exhibit a larger fluctuating width.
In order to cope with the situation, the catalyst deterioration detection apparatus for an internal combustion engine as disclosed in JP-A-5-98949 includes means for calculating the areas of patterns which are enclosed with the output signals of O2 sensors disposed on the upstream side and downstream side of a catalyst and predetermined signals (that is, the time integral values of the differences between the output signals of the O2 sensors disposed on the upstream side and downstream side and the predetermined signals), means for calculating cycles at which the outputs of the O2 sensors on the upstream side and downstream side are inverted with respect to the predetermined signals, means for calculating the deterioration decision parameter of the catalyst by using the calculated time integral values, the inversion cycle, or the combination of them, deterioration decision means for comparing the deterioration decision parameter with a predetermined value so as to decide the deterioration of the catalyst, and warning means for issuing warning in case of the decision of the deterioration.
Besides, the catalyst deterioration diagnosis apparatus for an internal combustion engine as disclosed in JP-A-7-305623 consists in including frequency calculation means for calculating the output signal frequencies of O2 sensors on the upstream side and downstream side of a catalyst, respectively, and means for subjecting the output signal of the downstream-side O2 sensor to filter processing on the basis of the frequency of the output signal of the upstream-side O2 sensor, wherein the influence of that fluctuation of a low frequency which is attendant upon the deviation between a target air/fuel ratio and a theoretical air/fuel ratio and which occurs in accordance with the running condition of the internal combustion engine, is suppressed by employing the amplitude ratio or number-of-times-of-inversion ratio between the output signal of the downstream-side O2 sensor subjected to the filter processing and the output signal of the upstream-side O2 sensor before the filter processing, thereby to prevent an erroneous diagnosis in the case of a catalyst deterioration diagnosis.
As stated before, in order to hold the purifiability of the catalyst high, the air/fuel ratio needs to be controlled into the very narrow range in the vicinity of the theoretical air/fuel ratio.
Especially, the upstream-side O2 sensor is directly influenced by the heat of the exhaust gas or the sulfur poisoning, and it is therefore liable to undergo the lowering of a response rate and the lowering of an output voltage due to the deterioration.
In general, therefore, the output signal of the downstream-side O2 sensor is employed, not only for the catalyst deterioration diagnosis, but also for the correction of the bias of the whole air/fuel ratio for the air/fuel ratio feedback control based on the output signal of the upstream-side O2 sensor, and so forth.
More specifically, the rich/lean decision voltage on the upstream side is corrected in accordance with the deviation between the output of the downstream-side O2 sensor and a downstream-side target voltage, whereby the influence of the deterioration of the upstream-side O2 sensor is compensated, and the air/fuel ratio is controlled into a state capable of holding the purifiability of the catalyst high.
FIG. 14 is a graph showing the output characteristic of a general O2 sensor in the related art, and the output waveforms of the O2 sensor during an air/fuel ratio feedback control.
As shown in FIG. 14, the output characteristic of the O2 sensor is a nonlinear characteristic versus an air/fuel ratio (oxygen concentration).
It is accordingly understood that, in a case (b) where a control range has been shifted in a rich (lower oxygen concentration) direction relative to an output voltage waveform (center characteristic) in the case where the air/fuel ratio feedback control is proceeding within a certain range (a) centering round the theoretical air/fuel ratio, and in a case (c) where the control range has been shifted in a lean (excessive oxygen concentration) direction, the output voltage waveform is distorted to narrow an amplitude, in spite of the same fluctuating width of the air/fuel ratio (oxygen concentration).
FIG. 15 is a graph for explaining a problem in the related art catalyst diagnosis for an internal combustion engine.
In a case where a deviation has occurred between the output characteristics of the upstream-side and downstream-side O2 sensors, under the influence of the deterioration of the O2 sensor or the discrepancy of the output characteristics, the output signal amplitude “ΔV_F2” and area equivalent value “S_F2” of the upstream-side O2 sensor in the case where an air/fuel ratio control range on the upstream side has shifted into the rich (higher output voltage) direction become smaller as compared with the output signal amplitude “ΔV_F1” and area equivalent value “S_F1” of the upstream-side O2 sensor in the case where the air/fuel ratio control range is controlled in the vicinity of the theoretical air/fuel ratio, respectively, as shown in FIG. 15 by way of example.
It is therefore understood that the amplitude ratio (ΔV_R1/ΔV_F2) and area equivalent value ratio (S_R1/S_F2) of the O2 sensor output signals on the upstream side and downstream side become larger as compared with the amplitude ratio (ΔV_R1/ΔV_F1) and area equivalent value ratio (S_R1/S_F1) in the case where the air/fuel ratio control range is controlled in the vicinity of the theoretical air/fuel ratio, respectively.
Accordingly, in the apparatus wherein the deteriorated state of the catalyst is diagnosed by employing the amplitude ratio or area equivalent value ratio of the O2 sensor output signals on the upstream side and downstream side, or a value obtained by combining the ratios, the deteriorated state cannot be detected, or the deterioration is erroneously diagnosed, in the worst case.
By the way, in the above description, the “area equivalent value” signifies a “value which is equivalent to an area enclosed with the O2 sensor output signal and a predetermined signal”, and this area equivalent value is the “time integral value of the difference between the O2 sensor output signal and the predetermined signal”.
Besides, the “predetermined signal” signifies, for example, a “voltage level in the vicinity of the amplitude center of the output signal of the O2 sensor” as indicated by a dot-and-dash line in FIG. 15.
Besides, in FIG. 15, “S_F1” and “S_F2” indicate the area equivalent values in the upstream-side O2 sensor output, and the value “S_F1” is the area equivalent value in the case where the O2 sensor output is oscillating in the vicinity of the center of the O2 sensor output characteristic, while the value “S_F2” is the area equivalent value in the case where the O2 sensor output is oscillating at a position which has shifted onto a higher voltage side from the vicinity of the center of the O2 sensor output characteristic.
In general, a target voltage for the output signal of the downstream-side O2 sensor is set so as to establish an air/fuel ratio state capable of holding the purifiability of the catalyst high, in accordance with the running condition of the internal combustion engine. In some cases, however, the target voltage is controlled in a region which is slightly shifted into the rich or lean direction relative to the theoretical air/fuel ratio.
As a result, in a case where an air/fuel ratio feedback control has been performed in a range in which the amplitude of an output voltage becomes small with respect to the output characteristic of the O2 sensor, the absolute value of the amplitude or area equivalent value of an O2 sensor output to be obtained becomes smaller as compared with a case where the air/fuel ratio feedback control is proceeding in a range centering round the theoretical air/fuel ratio.
Therefore, the amplitude ratio or area equivalent value ratio of the O2 sensor output signals on the upstream side and downstream side, or the value obtained by combining the ratios fluctuates greatly due to the slight deviation of the O2 sensor output characteristics on the upstream side and downstream side, and robustness against the erroneous diagnosis cannot be satisfactorily ensured under such a running condition in some cases.
Incidentally, the word “robust” is used in the significance that “control specifications are always satisfied however the features of a controlled system may fluctuate” in the field of control engineering.
Here, the word “robust” signifies an “immunity or strength” which prevents the erroneous diagnosis even in a case where the detection or diagnosis apparatus has undergone a disturbance such as the change of the running state of the internal combustion engine or the target air/fuel ratio (target voltage of the O2 sensor output).